In this paper we present full automation of the resistance bridge calibrator (RBC). The device is employed to calibrate AC and DC resistance bridges employed in primary thermometry. It consists of a resistor network made of 4 main resistors from which 35 different resistance values can be realized by switching 8 toggle switches on the calibrator panel. Up to 70 resistance ratios can be measured when the standard resistor and calibrator are interchanged. Literature shows that the resistor temperature coefficient may still effect measurement results. Placing the calibrator in a thermally stable environment can reduce it, but does not solve the problem of manual switching the calibrator switches. Our objective was to ensure full automation of the calibrator in which a computer would control the calibrator and receive the measurements from the bridge. Computer operation would completely substitute manual operation during which an operator has to be present, meaning that, repeated calibration measurements would be carried out with no active involvement of the personnel (at night or during weekends). We first set up a mechanical interface for manipulating the RBC switches. For this purpose we connected eight low torque servomotors to the switches via semi-rigid connector links, thus avoiding the switches from being mechanically harmed. We then made an interface between the servomotors and the computer. The interface was constructed around a simple 8-bit microcontroller. It enables different levels of commands to be sent from the computer to the mechanical unit. The interface also provides power for the motors which can be switched off during measurements. Finally we developed a control application to control the calibration process. In this application, end positions of switches are calibrated thus enabling the system to be used for different calibrators. The program switches combinations, gathers measurements from the bridge and calculates nonlinearity of the bridge. The measurements show that automation does not affect the long-term stability and mechanical repeatability of calibrator switches. Since no operator manipulation is needed, the whole system is inserted into a stryrofoam enclosure. Unlike in manual switching, where the enclosure has to be opened to switch between combinations, automated switching enables the enclosure to be closed throughout the process, making the calibrator environment thermally more stable. This allows for a lower uncertainty of measurements.
